A conventional automatic transmission control system is provided therein with a shift map defined by a throttle operating amount and an engine rotating number (or a vehicle speed) for each of gears of a transmission, and a shift point is determined by this shift map. According to the shift map, the rotating number of an engine (or vehicle speed) is set such that when the throttle operating amount by the driver is large, downshift is likely to occur but upshift is not likely to occur, and when the throttle operating amount is small, downshift is not likely to occur but upshift is likely to occur.
Further, in the conventional art, plural sorts of such shift maps are preliminarily prepared, and the optimal one of them is selected in accordance with the running state of the vehicle. For example, in the shift map suitable for upshift or downshift for hill climbing, the engine rotating number is set higher, and therefore the downshift timing is quickened so that a driving force for driving uphill is set greater. The running state of the above-described case is determined by assumption from the throttle operating amount and the engine rotating number for the gear position taken. For example, when an increase in the engine rotating number is small with respect to the predetermined throttle operating amount, it is assumed that the vehicle is in hill climbing, and a shift map which is set for the hill climbing is selected. Such a technique is described in “Nissan Diesel Technical Report No.60” by Nissan Diesel Motor Co., Ltd. on Mar. 14, 1998, pages 16 to 21.
In such a conventional automatic transmission control system, however, when the increase in the engine rotating number is smaller than the predetermined throttle operating amount, it is determined that the vehicle is hill climbing, and the shift map which is set for the hill climbing is selected. Therefore, since it is determined that the vehicle is in hill climbing and the shift map for the hill climbing is selected after the vehicle actually started hill climbing, the downshift timing is delayed, and the driving force of the engine becomes occasionally insufficient. Further, it is difficult to foresee the length of the uphill road and its gradient change, and downshift operation is carried out in some cases although the vehicle can sufficiently run uphill with the currently selected gear.
In the condition of a heavy commercial vehicle such as a truck and a bus, since the vehicle weight is heavy and margin of the engine driving force is small, there is often a case where a driver constantly depresses the accelerator pedal to a full amount during the driving. Hence, unnecessary downshift automatically occurs even during running on a flat road, and the upshift is delayed in some cases. Thus, the vehicle running at a high engine rotation lasts for a long while despite the driver's intention and as a result, the driver is unable to drive the vehicle economically.
Therefore, it is an object of the present invention to provide an automatic transmission control system in which the above-described problems can be overcome, and a driver can realize a point where a shift map is switched by an operation to depress an accelerator pedal, to thereby prevent occurrence of any unnecessary downshift and delay of upshift while enabling it to efficiently drive a vehicle.